How to Measure Low Concentration Gases in the Oil & Gas Industry

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Petrochemicals – From Crude to Chemicals

Ethylene is one of the most important feedstock for the petrochemical industry. Since it is used in manufacturing a wide range of compounds, ethylene has stringent purity specifications. One important step in purifying the ethylene is measuring the acetylene in the acetylene converter unit. The unit consists of a series of reactors or a single vessel with multiple beds of catalyst, which react the acetylene with hydrogen to form ethylene.

It is critical that the acetylene be brought down to the proper levels as it exits the final reactor; otherwise, any acetylene present will follow the ethylene all the way to the final product stream. To avoid this, producers will divert it to flare, a very costly decision.

Measuring Acetylene...

Typical acetylene measurement ranges from 0 3,000 down to 0-5 ppm. Moisture and acetylene are measured in pure ethylene gas and typical measurement ranges vary from 0-5 to 0-10 ppm.

Ethylene has stringent purity specifications especially for polymer grade ethylene as ammonia poisons polyethylene catalysts. Sample points for ammonia in polymer grade ethylene are either in the ethylene plant product or in a polyethylene plant feed. In both cases, the purity of the ethylene must be verified.

...in Refinery Processes

The catalytic reformer unit of a modern refinery is an important process unit for converting lower octane naphtha streams into higher octane aromatic compounds. These chemical conversions are done in catalytic reactors that transform straight-chain C6-C8 compounds found in the naphtha into light aromatics such as benzene, toluene and xylenes (BTX). This high octane reformate can then be used in gasoline blending or for applications in chemical plants.

The depentanized naphtha feed is mixed with recycled hydrogen gas, preheated and passed through a series of reactors where the conversion to aromatics takes place. After the last reactor, a hydrogen separator strips out the hydrogen. and other light gases from the stream.

After the removal of hydrogen, the stream enters a stabilizer tower (also called a debutanizer) that removes the butanes and lighter gases with the reformate leaving the bottoms for gasoline blending or sent to a chemical plant. Refineries normally operate at about 5–50 ppm moisture in the hydrogen recycle stream. During catalyst regeneration moisture can be much higher.

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